In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper "stock") is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rollers. The belt, often referred to as a "forming fabric", provides a papermaking surface on the upper surface of its upper run, which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium to form a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity or vacuum located on the lower surface (i.e., the "machine side") of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a "press felt." Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a "batt" layer of the press felt. The paper is then transferred to a drier section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Typically, papermakers' fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing on a pin seamable flap on each end or a special foldback, then reweaving these into pin seamable loops. In a flat woven papermakers' fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In the second technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein, the terms "machine direction" (MD) and "cross machine direction" (CMD) refer, respectively, to a direction aligned with the direction of travel of the papermakers' fabric on the papermaking machine, and a direction parallel to the fabric surface and traverse to the direction of travel. Both weaving methods described hereinabove are well known in the art, and the term "endless belt" as used herein refers to belts made by either method.
Effective sheet and fiber support and an absence of wire marking are important considerations in papermaking, especially for the forming section of the papermaking machine, where the wet web is initially formed. Wire marking is particularly problematic in the formation of fine paper grades, as it affects a host of paper properties, such as sheet mark, porosity, see through, and pin holing. Wire marking is the result of individual cellulosic fibers being oriented within the paper web such that their ends reside within gaps between the individual threads or yarns of the forming fabric. This problem is generally addressed by providing a permeable fabric structure with a co-planar surface which allows paper fibers to bridge adjacent yarns of the fabric rather than penetrate the gaps between yarns. As used herein, "co-planar" means that the upper extremities of the yarns defining the paper forming surface are at substantially the same elevation, such that at that level there is presented a substantially "planar" surface. Accordingly, fine paper grades intended for use in carbonizing, cigarettes, electrical condensers, quality printing, and like grades of fine paper, have typically heretofore been formed on very finely woven or fine wire mesh forming fabrics.
Such finely woven forming fabrics, however, often are delicate and lack dimensional stability in either or both of the machine and cross machine directions (particularly during operation), leading to a short service life for the fabric. In addition, a fine weave may adversely effect drainage properties of the fabric, thus rendering it less suitable as a forming fabric.
One proposed solution for improving forming fabric durability without adversely impacting drainage is offered in U.S. Pat. No. 4,987,929 to Wilson. Wilson describes a papermakers' forming fabric which includes a base fabric layer having single float MD knuckles on the paper contacting surface. Additional fiber supporting CMD yarns are woven into this initial fabric layer; these additional CMD yarns are preferably of smaller diameter than the base fabric layer yarns. The additional fiber supporting CMD yarns are held in place between adjacent fabric layer CMD yarns by additional CMD locator yarns, which are generally of approximately the same diameter as the fiber supporting yarns. This concept is extended in U.S. Pat. No. 5,518,042 to Wilson, in which the additional CMD yarns serve as both locator and support yarns, as they are interwoven so that portions thereof reside on opposite surfaces of the fabric.
One potential shortcoming of these configurations in some fabrics is that, even with the locator yarns in place, the support yarns can shift toward one of the adjacent CMD yarns of the base fabric layer. When such a shift occurs, the gap between the support yarn and the other adjacent CMD yarn increases, so the support yarn can be less successful at providing the requisite "bridge" for cellulosic fibers between adjacent CMD yarns of the base fabric.